Electronic display currently is a primary means for rapid delivery of information. Television sets, computer monitors, instrument display panels, calculators, printers, wireless phones, handheld computers, etc. aptly illustrate the speed, versatility, and interactivity that are characteristic of this medium. Of the known electronic display technologies, organic light emitting devices (OLEDs) are of considerable interest for their potential role in the development of full color, flat-panel display systems that may render obsolete the bulky cathode ray tubes still currently used in many television sets and computer monitors.
Generally, OLEDs are comprised of several organic layers in which at least one of the layers can be made to electroluminesce by applying a voltage across the device (see, e.g., Tang, et al., Appl. Phys. Lett. 1987, 51, 913, and Burroughes, et al., Nature, 1990, 347, 359). When a voltage is applied across a device, the cathode effectively reduces the adjacent organic layers (i.e., injects electrons) whereas the anode effectively oxidizes the adjacent organic layers (i.e., injects holes). Holes and electrons migrate across the device toward their respective oppositely charged electrodes. When a hole and electron meet on the same molecule, recombination is said to occur and an exciton is formed. Recombination of the hole and electron is preferably accompanied by radiative emission, thereby producing electroluminescence.
Depending on the spin states of the hole and electron, the exciton which results from hole and electron recombination can have either a triplet or singlet spin state. Luminescence from a singlet exciton results in fluorescence whereas luminescence from a triplet exciton results in phosphorescence. Statistically, for organic materials typically used in OLEDs, about one quarter of the excitons are singlets and the remaining three quarters are triplets (see, e.g., Baldo, et al., Phys. Rev. B, 1999, 60, 14422). Until the discovery that there were certain phosphorescent materials that could be used to fabricate practical electro-phosphorescent OLEDs having a theoretical quantum efficiency of up to 100% (i.e., harvesting all of both triplets and singlets), the most efficient OLEDs were typically based on materials that fluoresced. These materials fluoresced with a maximum theoretical quantum efficiency of only 25% (where quantum efficiency of an OLED refers to the efficiency with which holes and electrons recombine to produce luminescence), since the triplets to ground state transition is formally a spin forbidden process. Electro-phosphorescent OLEDs have now been show to have superior overall device efficiencies as compared with electro-fluorescent OLEDs (see, e.g., Baldo, et al., Nature, 1998, 395, 151 and Baldo, e.g., Appl. Phys. Lett. 1999, 75(3), 4).
Typically, OLEDs contain several thin organic layers between a hole injecting anode layer, comprising an oxide material such as indium-tin oxide (ITO), Zn—In—SnO2, SbO2, or the like, and an electron injecting cathode layer, comprising a metal layer such as Mg, Mg:Ag, or LiF:Al. An organic layer residing in proximity to the anode layer is usually referred to as the “hole transporting layer” (HTL) due to its propensity for conducting positive charge (i.e., holes). Various compounds have been used as HTL materials. The most common materials consist of various triaryl amines which show high hole mobilities. Similarly, the organic layer residing in proximity to the cathode layer is referred to as the “electron transporting layer” (ETL) due to its propensity to conduct negative charge (i.e., electrons). There is somewhat more variety in the ETL materials used in OLEDs as compared with for the HTL. A common ETL material is aluminum tris(8-hydroxyquinolate) (Alq3). Collectively, the ETL and HTL are often referred to as carrier layers. In some cases, an additional a layer may be present for enhancing hole or electron injection from the electrodes into the HTL or ETL, respectively. These layers are often referred to as hole injecting layers (HILs) or electron injecting layer (EIL). The HIL may be comprised of a small molecule such as 4,4′,4″-tris(30methylphenylphenylamino)triphenylamine (MTDATA) or polymeric material such as poly(3,4-ethylenedioxythiophene) (PEDOT). The EIL may be comprised of a small molecule material such as, e.g., copper phthalocyanine (CuPc). Many OLEDs further comprise an emissive layer (EL), or alternatively termed, luminescent layer, situated between the ETL and HTL, where electroluminescence occurs. Doping of the luminescent layer with various luminescent materials has allowed fabrication of OLEDs having a wide variety of colors.
In addition to the electrodes, carrier layers, and luminescent layer, OLEDs have also been constructed with one or more blocking layers to help maximize efficiency. These layers serve to block the migration of holes, electrons, and/or excitons from entering inactive regions of the device. For example, a blocking layer that confines holes to the luminescent layer effectively increases the probability that holes will result in a photoemissive event. Hole blocking layers desirably have a deep (i.e., low) HOMO energy level (characteristic of materials that are difficult to oxidize) and conversely, electron blocking materials generally have a high LUMO energy level. Exciton blocking materials have also been shown to increase device efficiencies. Triplet excitons, which are relatively long-lived, are capable of migrating about 1500 to 2000 Å, which is sometimes greater than the entire width of the device. An exciton blocking layer, comprising materials that are characterized by a wide band gap, can serve to block loss of excitons to non-emissive regions of the device.
In seeking greater efficiencies, devices have been experimentally created with layers containing light emitting metal complexes. Functional OLEDs having emissive layers of tris(2,2′-bipyridine)ruthenium(II) complexes or polymer derivatives thereof have been reported in Gao, et al., J. Am. Chem. Soc., 2000, 122, 7426, Wu, et al., J. Am. Chem. Soc. 1999, 121, 4883, Lyons, et al., J. Am. Chem. Soc. 1998, 120, 12100, Elliot, et al., J. Am. Chem. Soc. 1998, 120, 6781, and Maness, et al., J. Am. Chem. Soc. 1997, 119, 3987. Iridium-based and other metal-containing emitters have been reported in, e.g., Baldo, et al., Nature, 1998, 395, 151; Baldo, et al., Appl. Phys. Lett., 1999, 75, 4; Adachi, et al., Appl. Phys. Lett., 2000, 77, 904; Adachi, et al., Appl. Phys. Lett., 2001, 78, 1622; Adachi, et al., Bull. Am. Phys. Soc. 2001, 46, 863, Wang, et al., Appl. Phys. Lett., 2001, 79, 449, and U.S. Pat. Nos. 6,030,715; 6,045,930; and 6,048,630. Emissive layers containing (5-hydroxy)quinoxaline metal complexes as host material has also been described in U.S. Pat. No. 5,861,219. Efficient multicolor devices and displays are also described in U.S. Pat. No. 5,294,870 and International Application Publication No. WO 98/06242.
A metal-containing blocking layer has also been reported. Specifically, (1,1′-biphenyl)-4-olato)bis(2-methyl-8-quinolinolato N1,O8)aluminum (BAlq) has been used as a blocking layer in the OLEDs reported in Watanabe, et al. “Optimization of driving lifetime durability in organic LED devices using Ir complex,” in Organic Light Emitting Materials and Devices IV, Kafafi, ed. Proceedings of SPIE Vol. 4105, p. 175 (2001).
Although OLEDs promise new technologies in electronic display, they often suffer from degradation, short life spans, and loss of efficiency over time. The organic layers can be irreversibly damaged by sustained exposure to the high temperatures typically encountered in devices. Multiple oxidation and reduction events can also cause damage to the organic layers. Consequently, there is a need for the development of new materials for the fabrication of OLEDs. Compounds that are stable to both oxidation and reduction, have high Tg values, and readily form glassy thin films are desirable. The invention described hereinbelow helps fulfill these and other needs.